Each of approximately 200 fuel assemblies in a modern pressurized water reactor (PWR) typically consists of a square array of fuel rods having external dimensions on the order of 8.times.8.times.160 inches (20.times.20.times.410 centimeters). In order to maximize neutron economy, it is highly desirable to make all structural components of the assembly from Zircaloy. In contrast, the reactor vessel internal structures, which support the fuel assemblies, are typically made from type 304 stainless steel.
During reactor operation, forces acting on the fuel assemblies tend to cause small lateral distortions of the fuel assembly structures. The only upper limit on the total magnitude of such distortions is the summation of the lateral clearances between the fuel assemblies. Assemblies having all Zircaloy structures are more susceptible to such deformation than those having stainless steel or Inconel structures because Zircaloy has a lower coefficient of expansion than the structures which position the upper and lower ends of the assemblies, thereby causing an increase of lateral clearance of as much as 50% in going from room temperature to operating temperature. Such distortions are undersirable because they may complicate refueling and they may introduce slight variations in local power density by virtue of the uneven water gap between assemblies.
One practice is to modify the grid constructions located at various planer levels down the length of the fuel assemblies to maintain dimensional stability and positional relationship between adjacent fuel assemblies. Regardless of what success may be obtained by grid modification in material and form, the present invention is directed toward obtaining the stability required by the use of spring structure exerting lateral forces which can be actuated upon closure of the nuclear vessel. This spring structure will be provided at locations which will give mechanical support between fuel assemblies when in use, and release the assemblies from spring contact with each other when the reactor vessel is open for inspection and replacement of fuel assemblies.